Wireless LAN communication device and beacon transmitting method

ABSTRACT

A wireless LAN communication device is provided for making it possible to set beacons including DTIM information elements not to be synchronous in the case that the timing for generating the beacons including DTIM information elements is the same among access points, so that it is avoidable that only a terminal device under a specific access point among access points using the same channel is subjected to a delay and jitters, while anxiety about mutual interference of the terminal devices is removed in the case of broadcast and multicast service, so that a fair broadcast and multicast service can be received. In the device, a wireless LAN control unit ( 202 ) detects beacons of other stations received in a prescribed period of time before the transmission of its own station ( 200 ). A beacon analyzing unit ( 203 ) analyzes the beacons of the other stations and adjusts transmission intervals of its own DTIM beacon.

TECHNICAL FIELD

The present invention particularly relates to a wireless LANcommunication apparatus and a beacon transmission method for providing abroadcast/multicast service, through a plurality of access points, toterminals under the control of the respective access points.

BACKGROUND ART

As terminals mounted with a wireless LAN interface are becomingwidespread, it is becoming important to perform power saving control forterminals operating on a battery as a power supply and the wireless LANstandard provides a power management function to suppress the batteryconsumption of the terminals (e.g., Non-Patent Document 1). Furthermore,the wireless LAN standard provides radio access control methods such asDCF (Distributed Coordination Function), whereby each access pointperforms access control in an autonomous and distributed manner, and PCF(Point Coordination Function), whereby a specific station performsaccess control through polling in a concentrated manner.

The beacon period and DTIM (Delivery Traffic Indication Message) periodare provided as general settable parameters of access points. Here, the“beacon period” refers to the period in which a beacon is transmitted.Furthermore, the “DTIM period” refers to the period in which a beaconincluding information indicating that packet data that has been waitingto be transmitted is transmitted (hereinafter “DTIM beacon”), that is,the period in which a DTIM beacon including information indicating thattraffic stored at an access point is transmitted, and is expressed inbeacon period units. Furthermore, data transmitted after a DTIM beaconincludes a broadcast message or multicast message. When, for example,when the DTIM period is “3,” this means that a DTIM beacon istransmitted once every three beacons. Whether or not a beacontransmitted is a DTIM beacon is determined by whether or not the DTIMcount value in a TIM information element, which is an informationelement included in the beacon transmitted, is “0.”

The beacon period and DTIM period can be changed during operations, butthese periods are generally operated at their default values.Furthermore, in areas where wireless LAN is currently widespread,neighboring access points may communicate with each other using the samechannel. When neighboring access points carry out transmission using thesame channel, interference is avoided by controlling transmission offrames using a collision avoidance mechanism in wireless LANcommunication called “CSMA/CA.”

Next, power saving control using DCF will be explained using FIG. 1.FIG. 1 shows an overview of power saving control in an infrastructuremode. In FIG. 1, an access point transmits beacon 10 or DTIM beacon 11at preset beacon period H1 to inform terminal A and terminal B of thepresence of the access point. Beacon 10 and DTIM beacon 11 include a TIMelement indicating that frames directed to terminal A and terminal Bunder the control are accumulated, so that each terminal can recognizethat there are frames directed to the terminal, by referring to the TIMinformation element upon receiving beacon 10 and DTIM beacon 11.Terminal A and terminal B can set two types of states, the “awake” statein which power is supplied to a radio transmitting/receiving section andthe “doze” state in which only minimum necessary power is supplied, andcan determine the timing to set to the awake state using “ListenInterval” and “Receive DTIMs”. “Listen Interval” defines the period inwhich terminal A and terminal B receive a beacon, that is, beacon periodH1, and “Receive DTIM” indicates whether or not to receive DTIM beacon11. Since terminal A and terminal B in a power saving mode that receivea broadcast/multicast service must necessarily receive DTIM beacon 11,terminal A and terminal B transition to awake state 15 at DTIM periodH2. Furthermore, the access point transmits stored broadcast/multicastframe 12 to terminals under the control immediately after transmittingDTIM beacon 11. By changing the setting to the awake state or the dozestate in this way, each terminal can reduce power consumption comparedto a case where power is always kept ON. Upon receiving PS-poll 13transmitted from terminal A and terminal B, the access point transmitsunicast data 14 to terminal A or terminal B that transmitted PS-poll 13.

Furthermore, a delay occurs when power consumption is reduced throughthe power management function of radio terminals as described above, andtherefore prior arts are known which attempt to solve the problem ofdelay by adjusting the DTIM period for applications requiring real-timeperformance (e.g., Patent Document 1).

-   Non-Patent Document 1: IEEE Documents Part 11: Wireless LAN Medium    Access Control (MAC) and Physical Layer (PHY) Specifications-   Patent Document 1: Japanese Patent Application Laid-Open No.    2004-128949

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, in a conventional apparatus in a power saving mode defined inthe IEEE802.11 standard, when a plurality of neighboring radio accesspoints using the same radio channel use the same beacon period and DTIMperiod, the access points may transmit broadcast/multicast frames at thesame timing. In this case, depending on the beacon transmission timing,there can be a situation in which interference betweenbroadcast/multicast frames is likely to occur and the throughputs andservice delays of terminals receiving the service under the control ofthe respective access points may deteriorate. Furthermore, depending onalso services between access points, there can be differences in delayor bands available, resulting in problems that the service may bedelivered unfairly and the service quality of the broadcast/multicastservice may deteriorate.

FIG. 2 shows a situation in which interference betweenbroadcast/multicast frames occurs. In FIG. 2, access point A iscommunicating with terminal A and access point B is communicating withterminal B. When beacon period H3 of access point A is substantially thesame as beacon period H4 of access point B and timing at which accesspoint A transmits beacon 20 is slightly earlier than timing at whichaccess point B transmits beacon 21, access point B slightly delaystiming of transmitting a beacon with respect to the transmission frameof access point A according to a communication collision avoidancemechanism in the wireless LAN.

For example, DTIM beacon 23 transmitted by access point B shouldoriginally be transmitted at the timing of DTIM beacon 26, but thetiming is delayed to timing after access point A transmitsbroadcast/multicast frame 25. Likewise, broadcast/multicast frame 24transmitted by access point B should originally be transmitted at timingof broadcast/multicast frame 27, but the timing is delayed to timingafter access point A transmits broadcast/multicast frame 25 and afteraccess point B transmits DTIM beacon 23 to be transmitted.

As a result, when the timing access point A transmits DTIM beacon 22 isthe same as the timing access point B transmits DTIM beacon 23, there isa problem that transmission of broadcast/multicast frame 24 by accesspoint B is always after transmission of broadcast/multicast frame 25 byaccess point A and broadcast/multicast frame 24 received by terminal Bhas a greater delay and jitter than broadcast/multicast frame receivedby terminal A, and the band used also deteriorates. This problem becomesparticularly obvious when access point A and access point Bsimultaneously transmit their services using the samebroadcast/multicast frame to terminals.

Furthermore, Patent Document 1 describes a technique adopting an optimumDTIM period to terminals for each application and thereby improvingdelay and enabling realtime performance and power saving controltogether. However, Patent Document 1 does not assume the presence of aplurality of access points, takes no account of interference between theaccess points, and therefore when terminals under the control of aplurality of access points use the same application, the same DTIMperiod is set for these access points. As a result, DTIM beacons aresynchronized with each other among a plurality of access points, whichresults in a problem that only terminals under the control of a certainspecific access point have an increased delay and jitter and mutualinterference may occur at the time of a broadcast/multicast servicebetween access points.

It is therefore an object of the present invention to provide a wirelessLAN communication apparatus and a beacon transmission method thatprevent, when the timing to transmit DTIM beacons is substantially thesame between access points, the DTIM beacons from synchronizing witheach other, and can thereby prevent, even between access points usingthe same channel, only terminals under the control of a certain specificaccess point from having an increased delay and jitter, eliminate thepossibility of mutual interference during a broadcast/multicast serviceand allow the terminals to receive a fair broadcast/multicast service.

Means for Solving the Problem

The wireless LAN communication apparatus of the present invention adoptsa configuration including: a receiving section that receives a beacon,the beacon including first transmission period information comprisinginformation about a beacon transmission period at another communicationapparatus and second transmission period information comprisinginformation about a transmission period of a delivery traffic indicationmessage beacon at the another communication apparatus, the deliverytraffic indication message beacon including information indicating thatpacket data that has been waiting to be transmitted is transmitted; abeacon detection section that detects the beacon received in thereceiving section in a predetermined time immediately before or after abeacon of the wireless local area network communication apparatus istransmitted; a beacon correction section that corrects a transmissionperiod of the delivery traffic indication message beacon at the wirelesslocal area network communication apparatus based on the firsttransmission period information and the second transmission periodinformation included in the detected beacon; and a transmitting sectionthat transmits the delivery traffic indication message beacon at thecorrected transmission period and transmits the packet data immediatelyafter having transmitted the delivery traffic indication message beacon.

Furthermore, the wireless LAN communication apparatus of the presentinvention adopts a configuration including: a receiving section thatreceives a delivery traffic indication message beacon, the beaconincluding information indicating that packet data that has been waitingto be transmitted is transmitted; a beacon detection section thatdetects the beacon received in the receiving section in a predeterminedtime immediately before or after the beacon of the wireless local areanetwork communication apparatus is transmitted; a beacon correctionsection that corrects the transmission period of the delivery trafficindication message beacon at the wireless local area networkcommunication apparatus when the detection section detects reception ofthe beacon; and a transmitting section that transmits the deliverytraffic indication message beacon at the corrected transmission periodand transmits the packet data immediately after having transmitted thedelivery traffic indication message beacon.

The beacon transmission method of the present invention includes:receiving a beacon including first transmission period informationcomprising information about a beacon transmission period at anothercommunication apparatus and second transmission period informationcomprising information about a transmission period of a delivery trafficindication message beacon at the another communication apparatus, thedelivery traffic indication message beacon including informationindicating that packet data that has been waiting to be transmitted istransmitted; detecting a beacon received in a predetermined timeimmediately before or after the beacon of the wireless local areanetwork communication apparatus is transmitted; correcting atransmission period of the delivery traffic indication message beacon atthe wireless local area network communication apparatus based on thefirst transmission period information and the second transmission periodinformation included in the detected beacon; and transmitting thedelivery traffic indication message beacon at the corrected transmissionperiod.

Furthermore, the beacon transmission method of the present inventionincludes: receiving a delivery traffic indication message beaconincluding information indicating that packet data that has been waitingto be transmitted is transmitted; detecting a beacon received in apredetermined time immediately before or after the beacon of thewireless local area network communication apparatus is transmitted;correcting the transmission period of the delivery traffic indicationmessage beacon at the wireless local area network communicationapparatus when the reception of the beacon is detected; and transmittingthe delivery traffic indication message beacon at the correctedtransmission period.

Advantageous Effect of the Invention

When the timing to transmit the DTIM beacon is substantially the samebetween access points, the present invention prevents DTIM beacons fromsynchronizing with each other, so that, even between access points usingthe same channel, it is possible to prevent the delay and jitter of onlythe terminals under the control of a certain specific access point fromincreasing, eliminate the possibility of interfering with each otherduring a broadcast/multicast service and receive a broadcast/multicastservice fairly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows an overview of power saving control in a conventionalinfrastructure mode;

FIG. 2 shows a situation in which interference occurs betweenconventional broadcast/multicast frames;

FIG. 3 illustrates an overview of a broadcast/multicast serviceaccording to Embodiment 1 of the present invention;

FIG. 4 is a block diagram showing a configuration of a wireless LANcommunication apparatus according to Embodiment 1 of the presentinvention;

FIG. 5 is a flowchart showing operations of the wireless LANcommunication apparatus according to Embodiment 1 of the presentinvention;

FIG. 6 shows beacon reception timing according to Embodiment 1 of thepresent invention;

FIG. 7 shows a data format of a TIM information element according toEmbodiment 1 of the present invention;

FIG. 8 is a flowchart showing a method of correcting a beacontransmission period according to Embodiment 1 of the present invention;

FIG. 9 shows a method of correcting a beacon transmission periodaccording to Embodiment 1 of the present invention;

FIG. 10 is a block diagram showing a configuration of a wireless LANcommunication apparatus according to Embodiment 2 of the presentinvention;

FIG. 11 is a flowchart showing operations of the wireless LANcommunication apparatus according to Embodiment 2 of the presentinvention;

FIG. 12 shows beacon reception timing according to Embodiment 2 of thepresent invention;

FIG. 13 is a flowchart showing a method of correcting a beacontransmission period according to Embodiment 3 of the present invention;and

FIG. 14 shows the method of correcting a beacon transmission periodaccording to Embodiment 3 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained below in detailwith reference to the accompanying drawings.

Embodiment 1

FIG. 3 illustrates an overview of a broadcast/multicast serviceaccording to Embodiment 1 of the present invention. As shown in FIG. 3,broadcast/multicast service delivery data stored in content server 101is delivered to terminal apparatuses 104 under the control of respectiveaccess points 103 via Internet 102 and the plurality of access points103.

FIG. 4 is a block diagram showing a configuration of wireless LANcommunication apparatus 200 according to Embodiment 1 of the presentinvention. Wireless LAN communication apparatus 200 is, for example,access point 103 in a wireless LAN.

Radio transmitting/receiving section 201 receives a beacon transmittedfrom another access point, down-converts the received beacon and outputsthe beacon to wireless LAN control section 202. Furthermore, radiotransmitting/receiving section 201 up-converts a beacon received asinput from wireless LAN control section 202 and transmits the beacon.

Wireless LAN control section 202, which is a beacon detection section,performs control over a MAC layer such as CSMA/CA and access control,detects a beacon received as input from radio transmitting/receivingsection 201, which radio transmitting/receiving section 201 has receivedwithin a predetermined time, and outputs the detection result to beaconanalysis section 203. Furthermore, wireless LAN control section 202outputs such a DTIM beacon that is transmitted at a DTIM periodcommanded by beacon correction section 204 to radiotransmitting/receiving section 201.

Beacon analysis section 203, which is a beacon correction means,analyzes a beacon transmitted from another station using the detectionresult received as input from wireless LAN control section 202. That is,beacon analysis section 203 commands beacon correction section 204 tocorrect the DTIM period for wireless LAN communication apparatus 200,based on beacon period information (i.e. first transmission periodinformation), which is information about the beacon period, and DTIMperiod information (i.e. second transmission period information), whichis information about the DTIM period, included in the beacon detected bywireless LAN control section 202. To be more specific, when the beaconperiod of another station in the beacon period information matches withthe beacon period of wireless LAN communication apparatus 200 and theDTIM period of another station in the DTIM period information matcheswith the DTIM period of wireless LAN communication apparatus 200, beaconanalysis section 203 commands to correct the DTIM period of wireless LANcommunication apparatus 200, such that the DTIM period of anotherstation does not match with the DTIM period of wireless LANcommunication apparatus 200.

Beacon correction section 204, which is a beacon correction means,commands wireless LAN control section 202 to transmit DTIM beacons atthe DTIM period after the correction commanded from beacon analysissection 203. Details of the method of correcting a DTIM period will bedescribed later.

Next, operations of wireless LAN communication apparatus 200 will beexplained using FIG. 5. FIG. 5 is a flowchart showing operations ofwireless LAN communication apparatus 200. In FIG. 5, suppose wirelessLAN communication apparatus 200 is the local station and is access pointA, and wireless LAN communication apparatus 200 is another station andis access point B.

First, wireless LAN control section 202 of access point A checks whetheror not a beacon has been received during a beacon monitoring period(step ST501). Here, the “beacon monitoring period” refers to the periodimmediately before or after access point A transmits a beacon.

FIG. 6 shows an example where beacon monitoring period T601 of accesspoint A is provided immediately before beacon transmission by accesspoint A. That is, the fact that beacon 602 received by access point A iswithin beacon monitoring period T601 means that there is a possibilitythat the transmission timing of beacon 603 of access point A may overlapthe transmission timing of beacon 602 of access point B. The length ofbeacon monitoring period T601 may be arbitrarily set.

Referring back to FIG. 5, when the beacon is not within beaconmonitoring period T601 in step ST501, wireless LAN control section 202terminates the processing or moves to step ST502 when the beacon iswithin beacon monitoring period T601.

Next, beacon analysis section 203 of access point A extracts informationabout a beacon period (Beacon Period), DTIM count (DTIM Count) value andDTIM period (DTIM Period), from the acquired beacon (step ST502).

FIG. 7 shows a data format of TIM information elements included in abeacon. As shown in FIG. 7, the TIM information element includes ID(Element ID) 701, data length (Length) 702, DTIM count value (DTIMCount) 703, DTIM period (DTIM Period) 704, bitmap control (BitmapControl) 705 and partial virtual bitmap (Partial Virtual Bitmap) 706.Beacon analysis section 203 extracts DTIM count value 703 and DTIMperiod 704 from the TIM information element. For example, when the DTIMperiod is set to “3,” a DTIM beacon is transmitted every thirdtransmission. The DTIM count value indicates the number of beacons thatneed to be transmitted by the time the DTIM beacon is transmitted. Forexample, when wireless LAN communication apparatus 200 transmits a DTIMbeacon, the DTIM count value always becomes “0.” Here, DTIM count value“0” in the TIM information element included in the DTIM beacon is theinformation indicating that packet data that has been waiting to betransmitted is transmitted.

Referring back to FIG. 5 again, beacon analysis section 203 of accesspoint A then compares the parameters of the acquired beacon of accesspoint B with the parameters to be included in the beacon transmitted byaccess point A at the next timing. That is, beacon analysis section 203compares the beacon period and DTIM period included in the beacons ofboth access point A and access point B (step ST503). Beacon analysissection 203 terminates the processing when the comparison result showsthat the beacon periods are not the same, or moves to step ST504 whenthe beacon periods are the same.

Next, beacon analysis section 203 compares the DTIM periods (stepST504), and moves to step ST505 when the DTIM periods are the same, orterminates the processing when the DTIM periods are not the same.

Next, beacon analysis section 203 of access point A decides whether ornot both the acquired DTIM count value of access point B and the nextDTIM count value to be transmitted by access point A are “0” (stepST505). When both DTIM count values are “0,” beacon analysis section 203moves to step ST506 or terminates the processing when both DTIM countvalues are not “0.”

Next, beacon correction section 204 of access point A corrects the DTIMperiod (step ST506), and access point A transmits a beacon so as tocorrespond to the corrected DTIM period.

Next, details of the method of correcting a DTIM period by beaconcorrection section 204 in step ST506 will be explained using FIG. 8 andFIG. 9. FIG. 8 and FIG. 9 are flowcharts showing the method ofcorrecting a DTIM period.

Beacon correction section 204 is waiting for a trigger from beaconanalysis section 203 (step ST801), and acquires, when the trigger isinputted, a DTIM count value of the next beacon to be transmitted fromwireless LAN control section 202 (step ST802).

Next, beacon correction section 204 decides whether or not the DTIMcount value acquired from wireless LAN control section 202 is “0” (stepST803). When the DTIM count value of the beacon is not “0,” beaconcorrection section 204 waits for transmission of the next beacon (stepST804) and returns to step ST802. On the other hand, when the DTIM countvalue of the beacon is “0,” beacon correction section 204 requestswireless LAN control section 202 to correct the DTIM count value of thenext beacon from “0” to “1” (step ST805) and returns to the step ofwaiting for a trigger from beacon analysis section 203 again (stepST801).

Next, the method of correcting a DTIM period will be explained infurther detail using FIG. 9. Beacons are transmitted from each accesspoint at regular intervals. Furthermore, when a beacon is transmitted,the DTIM count value is decremented by “1” every time a beacon istransmitted. The value subtracting “1” from the DTIM period is in thebeacon next to the DTIM beacon in which the DTIM count value is “0,”that is, the beacon next to the DTIM beacon.

For example, when the DTIM period is “3,” the DTIM count value changesfrom “2,” “1,” “0,” “2,” “1,” “0,”. . . in order, every time beacon 901to 905 is transmitted. With beacon 909 to be transmitted next to beacon908 having the first DTIM count value “1” after a startup, beaconcorrection section 204 transmits beacon 909 having a DTIM count value“1” again. Upon receiving beacons 908 and 909 having a DTIM count value“1,” the terminal recognizes that the next beacon to be received is aDTIM beacon and sets awake state 912 at the next beacon receivingtiming, too. Immediately after transmitting DTIM beacon 910 of DTIMcount value “0,” access point A transmits a broadcast/multicast frame tothe terminal. The above-described operations cause the transmissiontiming of DTIM beacon 903 by access point B to differ from thetransmission timing of DTIM beacon 910 by access point A, and canthereby prevent interference with mutual communication between accesspoint A and access point B during transmission of a broadcast/multicastframe. By the way, beacon analysis section 203 is effective only whenthe DTIM period is “2” or more.

As described so far, the present embodiment determines whether or notthe beacon period and the DTIM period synchronize with each otherbetween access points, corrects the DTIM period in the case ofsynchronization, and can thereby prevent, even among access points usingthe same channel, a delay and jitter of only terminals under the controlof a certain specific access point from increasing, eliminate thepossibility of mutual interference during a broadcast/multicast serviceand receive an impartial broadcast/multicast service.

Embodiment 1 sets a beacon monitoring period immediately beforetransmission of a beacon of wireless LAN communication apparatus 200,but the present invention is not limited to this, and the beaconmonitoring period may be set immediately after transmission of a beaconof wireless LAN communication apparatus 200 or the beacon monitoringperiod may also be set both immediately before and immediately aftertransmission of a beacon of wireless LAN communication apparatus 200.When the beacon monitoring period is set immediately after transmissionof a beacon of wireless LAN communication apparatus 200, in step ST505,the DTIM count value of the received beacon is compared with the DTIMcount value of the last beacon transmitted by wireless LAN communicationapparatus 200 instead of comparing the DTIM count value of the receivedbeacon with the DTIM count value of the next beacon to be transmitted bywireless LAN communication apparatus 200. Furthermore, in Embodiment 1,it is decided in step ST503 whether or not the beacon periods are thesame, but the present invention is not limited to this and the systemmay be adapted so as to decide in step ST503 whether or not the beaconperiod of wireless LAN communication apparatus 200 is an integermultiple of the beacon period of another access point and move to stepST504 when the beacon period of wireless LAN communication apparatus 200is an integer multiple of the beacon period of another access point.Furthermore, in Embodiment 1, it is decided in step ST504 whether or notthe DTIM periods are the same, but the present invention is not limitedto this and the system may also be adapted so as to decide in step ST504whether or not the DTIM period of wireless LAN communication apparatus200 is an integer multiple of a DTIM period of another access point andmove to step ST505 when the DTIM period of wireless LAN communicationapparatus 200 is an integer multiple of the DTIM period of anotheraccess point.

Furthermore, in Embodiment 1, the DTIM period is corrected bytransmitting beacons of DTIM count value “1” consecutively, but thepresent invention is not limited to this, and the DTIM period may becorrected by transmitting a beacon of an arbitrary DTIM count valueother than “0” next to the beacon having a DTIM count value of “1.” Thatis, the DTIM period can be corrected by transmitting an extra beaconhaving a DTIM count value different from a normal DTIM count value.

Embodiment 2

Embodiment 2 measures the number of times beacons are received andcorrects, when the measured number of times is equal to or greater thana predetermined value, only a DTIM period without correcting any beaconperiod.

FIG. 10 is a block diagram showing a configuration of wireless LANcommunication apparatus 1000 according to Embodiment 2 of the presentinvention.

Wireless LAN communication apparatus 1000 according to presentEmbodiment 2 corresponds to wireless LAN communication apparatus 200according to Embodiment 1 shown in FIG. 4 with counter 1001 added asshown in FIG. 10. In FIG. 10, parts having the same configurations asthose in FIG. 4 will be assigned the same reference numerals andexplanations thereof will be omitted.

Beacon analysis section 203 analyzes a beacon transmitted by anotherstation using a detection result received as input from wireless LANcontrol section 202. That is, beacon analysis section 203 decideswhether the beacon is a beacon received by radio transmitting/receivingsection 201 within a predetermined time before transmission of thebeacon for wireless LAN communication apparatus 1000 or a beaconreceived by radio transmitting/receiving section 201 within apredetermined time after transmission of the beacon for wireless LANcommunication apparatus 1000. Beacon analysis section 203 then counts upcounter 1001 according to the decision result. Furthermore, when thecount value on counter 1001 is equal to or greater than a threshold,beacon analysis section 203 commands beacon correction section 204 tocorrect the DTIM period of wireless LAN communication apparatus 1000based on the beacon period information and DTIM period informationincluded in the beacon detected by wireless LAN control section 202. Tobe more specific, when the beacon period of another station in thebeacon period information matches with the beacon period of wireless LANcommunication apparatus 1000 and the DTIM period of another station inthe DTIM period information matches with the DTIM period of wireless LANcommunication apparatus 1000, beacon analysis section 203 commands tocorrect the DTIM period of wireless LAN communication apparatus 1000such that the DTIM period of another station does not match with theDTIM period of wireless LAN communication apparatus 1000.

Counter 1001, which is a measuring means, measures the number of timesbeacons are detected according to the decision result of beacon analysissection 203.

Next, operations of wireless LAN communication apparatus 1000 will beexplained using FIG. 11. FIG. 11 is a flowchart showing operations ofwireless LAN communication apparatus 1000. In FIG. 11, suppose wirelessLAN communication apparatus 1000 is the local station and is accesspoint A and wireless LAN communication apparatus 1000 is another stationand is access point B. In FIG. 11, parts having the same configurationsas those in FIG. 5 will be assigned the same reference numerals andexplanations thereof will be omitted.

Beacon analysis section 203 of access point A decides whether or notboth the acquired DTIM count value of access point B and the next DTIMcount value to be transmitted by access point A are “0” (step ST505).When both DTIM count values are “0,” beacon analysis section 203 movesto step ST1101 or terminates the processing when both DTIM count valuesare not “0.”

Next, beacon analysis section 203 of access point A decides whether ornot a beacon detected by wireless LAN control section 202 is within abeacon monitoring period after transmission of the beacon of wirelessLAN communication apparatus 1000 (step ST1101). Here, the beaconmonitoring period in present Embodiment 2 is the periods immediatelybefore and immediately after access point A transmits a beacon.

FIG. 12 shows an example where beacon monitoring period T1201 (i.e.pre-transmission monitoring time) of access point A is providedimmediately before transmission of beacon 1204 of access point A, andbeacon monitoring period T1202 (i.e. post-transmission monitoring time)of access point A is provided immediately after transmission of beacon1204 of access point A. That is, the fact that beacon 1203 received byaccess point A is in beacon monitoring period T1201 or beacon monitoringperiod T1202 means that there is a possibility that the transmissiontiming of beacon 1204 of access point A overlaps the transmission timingof beacon 1203 of access point B. The period of beacon monitoring periodT1201 and beacon monitoring period T1202 can be set to an arbitrarylength.

Furthermore, counter 1001 includes a pre-transmission interferencecounter that counts when a beacon is detected within beacon monitoringperiod T1201 and a post-transmission interference counter that countswhen a beacon is detected within beacon monitoring period T1202.

Referring back to FIG. 11, the post-transmission interference counter ofcounter 1001 is counted up when a beacon is detected within beaconmonitoring period T1202 (step ST1102) and the pre-transmissioninterference counter of counter 1001 is counted up when a beacon isdetected within beacon monitoring period T1201 (step ST1103).

Next, beacon analysis section 203 of access point A decides whether ornot the count value on the post-transmission interference counter ofcounter 1001 is equal to or greater than a threshold X (X is an integerequal to or greater than “1”) (step ST1104).

When the count value on the post-transmission interference counter ofcounter 1001 is not equal to or greater than the threshold X, theprocessing is terminated, and, when the count value on thepost-transmission interference counter of counter 1001 is equal to orgreater than the threshold X, the beacon period of wireless LANcommunication apparatus 1000 is corrected (step ST506).

Furthermore, beacon analysis section 203 of access point A decideswhether or not the count value on the pre-transmission interferencecounter of counter 1001 is equal to or greater than a threshold Y (Y isan integer equal to or greater than “1”) (step ST1105).

When the count value on the pre-transmission interference counter ofcounter 1001 is not equal to or greater than the threshold Y, theprocessing is terminated and when the count value on thepre-transmission interference counter of counter 1001 is equal to orgreater than the threshold Y, the beacon period of wireless LANcommunication apparatus 1000 is corrected (step ST506).

Next, beacon correction section 204 of access point A corrects the DTIMperiod (step ST506) and access point A transmits the beacon so as tocorrespond to the corrected DTIM period.

Next, beacon analysis section 203 of access point A resets thepre-transmission interference counter and post-transmission interferencecounter of counter 1001, and sets the counter value to “0” (stepST1106). Even if the transmission period of the beacon is not corrected,when a certain time has passed, counter 1001 may be designed so as toset the count value on the pre-transmission interference counter and thecount value on the post-transmission interference counter to “0.”

Furthermore, the thresholds of the pre-transmission interference counterand the post-transmission interference counter of counter 1001 havedifferent values between X and Y. When the thresholds of thepre-transmission interference counter and the post-transmissioninterference counter of counter 1001 are “1,” the DTIM period iscorrected by detection of one beacon. When access point A detects thebeacon of access point B in beacon monitoring period T1202, the DTIMperiod is corrected when access point B detects access point A in beaconmonitoring period T1201 X times. Furthermore, when access point Asimultaneously detects access point A in beacon monitoring period T1201Y times, the same DTIM period is corrected. In this case, when X and Yhave the same value, beacons having the same DTIM count value aretransmitted at the same timing even after correction. Therefore, bysetting X and Y to different values, the access point having the value Xor Y, whichever is smaller, can correct the DTIM period first andprevent mutual interference between access point A and access point B.Furthermore, when access point B is an access point without performingthe mounting of present Embodiment 2, access point A corrects the beacontransmission period not only upon detecting a beacon in beaconmonitoring period T1201 but also upon detecting a beacon in beaconmonitoring period T1202. As a result, it is possible to prevent mutualinterference between access point A and access point B.

As shown above, in addition to the effect of Embodiment 1 above, presentEmbodiment 2 corrects the DTIM period when the number of times beaconsare detected reaches a predetermined number of times, and therefore bycausing the number of times beacons are detected to start correction ofthe DTIM period to vary from one access point to another, it is possibleto prevent the access points from performing the same correction causingtimings of transmitting DTIM beacons again to synchronize with eachother. That is, present Embodiment 2 can autonomously correct DTIMperiods irrespective of whether or not other access points are mountedwith the configuration of present Embodiment 2. As a result,interference between multicast/broadcast services can be avoided in anyenvironment and it is possible to improve service quality such as bandsand delays.

Embodiment 2 sets a beacon monitoring period immediately aftertransmission of a beacon of the own station, but the present inventionis not limited to this and the beacon monitoring period may be setimmediately before transmission of the beacon of the own station or thebeacon monitoring period may be set both immediately before andimmediately after transmission of the beacon of the own station. Whenthe beacon monitoring period is set immediately before transmission ofthe beacon of the own station, in step ST505, the DTIM count value ofthe received beacon is compared with the DTIM count value of the nextbeacon to be transmitted by the own station instead of comparing theDTIM count value of the received beacon with the count value of the lasttransmitted beacon. Furthermore, in step ST503, present Embodiment 2decides whether or not the beacon periods are the same, but the presentinvention is not limited to this, and in step ST503, the system may alsobe adapted so as to decide whether or not the beacon period of the ownstation is an integer multiple of the beacon period of another accesspoint, and move to step ST504 when the beacon period of the own stationis an integer multiple of the beacon period of another access point.Furthermore, in step ST504, present Embodiment 2 decides whether or notthe DTIM periods are the same, but the present invention is not limitedto this, and in step ST504, the system may also be adapted so as todecide whether or not the DTIM period of the own station is an integermultiple of the DTIM period of another access point and move to stepST505 when the DTIM period of the own station is an integer multiple ofthe DTIM period of another access point.

Furthermore, present Embodiment 2 corrects the DTIM period byconsecutively transmitting beacons of DTIM count value “1,” but thepresent invention is not limited to this and the DTIM period may also becorrected by transmitting beacons of an arbitrary DTIM count value otherthan “0” next to the beacon whose DTIM count value is “1.” That is, theDTIM period can be corrected by transmitting an extra beacon having aDTIM count value different from a normal DTIM count value. Furthermore,present Embodiment 2 assumes that the threshold X to be compared withthe count measured using the post-transmission interference counter hasa value different from the threshold Y to be compared with the countmeasured using the pre-transmission interference counter, but thepresent invention is not limited to this, and the threshold to becompared with the count measured using the post-transmissioninterference counter may have the same value as the threshold to becompared with the count measured using the pre-transmission interferencecounter.

Embodiment 3

Instead of correcting only the DTIM period without correcting the beaconperiod as in the case of Embodiment 1 and Embodiment 2 above, presentEmbodiment 3 shifts beacon transmission timing of the local stationaccording to a predetermined rule regardless of the beacon period, andthereby corrects both the beacon period and DTIM period. When allbeacons are DTIM beacons, that is, when the DTIM periods of all beaconsare “1,” the DTIM count value is always “0”, and therefore the techniqueof shifting the beacon period of the DTIM beacon cannot be used, and soa new approach is necessary.

FIG. 13 is a flowchart showing a method of correcting a DTIM periodaccording to Embodiment 3 of the present invention. In presentEmbodiment 3, all beacons transmitted by each wireless LAN communicationapparatus are DTIM beacons. In present Embodiment 3, the configurationof the wireless LAN communication apparatus is the same as theconfiguration of FIG. 4 and operations of the wireless LAN communicationapparatus are the same as those in FIG. 5, and therefore explanationsthereof will be omitted.

Beacon correction section 204 is waiting for a trigger from beaconanalysis section 203 (step ST1301) and corrects the beacon period of thenext beacon to be transmitted when the trigger is inputted (stepST1302). In this case, beacon correction section 204 corrects the beaconperiod using equation 1.X=A+A/(B+1)  (Equation 1)where: X is a beacon period after the correction;

-   -   A is the current beacon period before the correction; and    -   B is the number of access points detected.

Next, beacon correction section 204 requests wireless LAN controlsection 202 to transmit the next and subsequent beacons to betransmitted at corrected beacon period X calculated from equation 1(step ST1303), and waits for the beacons to be transmitted at correctedbeacon period X (step ST1304).

Next, the method of correcting a DTIM period will be explained infurther detail using FIG. 14. FIG. 14 shows the method of correcting aDTIM period. In FIG. 14, access point B is transmitting beacons 1401 to1404 at certain beacon period A. On the other hand, access point Atransmits beacons at beacon period A from beacon 1405 up till beacon1406. Furthermore, in FIG. 14, access point A assumes the beacon to betransmitted at the next timing to be beacon 1406 and assumes the beaconto be transmitted next to beacon 1406 to be beacon 1407.

Beacon correction section 204 starts correcting the beacon period at thetiming of transmitting beacon 1406 and corrects beacon period A aftertransmitting beacon 1406 until beacon 1407 is transmitted to beaconperiod X using equation 1. In this way, the beacon period between beacon1406 and beacon 1407 to be transmitted next to beacon 1406 becomescorrected beacon period X, so that it is possible to preventinterference that would be produced by being transmitted at timing ofbeacon P0 unless the beacon period is corrected.

Next, after beacon 1407 is transmitted, beacon correction section 204changes the beacon period between beacon 1407 and beacon 1408 fromcorrected beacon period X back to beacon period A before correction, andrequests wireless LAN control section 202 to transmit beacon 1408 atbeacon period A (step ST1305). In this way, beacons 1407 and 1408 ofaccess point A after correcting the beacon period are always transmittedwith a delay time α from the transmission timing of beacons 1403 and1404 of access point B, and mutual interference can be therebyprevented.

Embodiment 3 corrects X according to the number of access pointsdetected as shown in equation 1, but the present invention is notlimited to this, and it is possible to prevent interference with otheraccess points by setting the beacon period of access point A to anarbitrary length greater than the beacon period before the correction.Furthermore, when only one access point is detected or when there is nolikelihood that a plurality of access points interfere with each othersimultaneously, the corrected beacon period may be preferably set tohalf the period of the beacon period before the correction. For example,when the beacon period before the correction is 100 milliseconds, thecorrected beacon period is set to 150 milliseconds by shifting thecorrected beacon period from the beacon period before the correction by50 milliseconds.

Furthermore, as another method of correcting the beacon period, whenthere is one more access point of the same channel other than the localstation, the corrected beacon period is shifted by half the beaconperiod before the correction. For example, when the beacon period beforethe correction of access point A is 100 milliseconds, beacon correctionsection 204 of access point A transmits the next beacon after 50milliseconds pass from the timing the beacon of access point B isreceived.

Furthermore, as another method of beacon period correction, when thereare a plurality of access points the same channel and the same beaconperiod apart from the local station, access point A sets a beacon periodhaving the value given by dividing the beacon period of the localstation before the correction by the number of access points includingthe local station. For example, when the beacon period before thecorrection is 100 milliseconds, if there are two other access pointshaving the same beacon period on the same channel, the next beacon istransmitted at a beacon period shifted by 100 milliseconds/3 (the totalnumber of access points including the own station is 3), that is, 33milliseconds.

In addition to the above-described effect of Embodiment 1, presentEmbodiment 3 can reduce interference between broadcast/multicast frameseven when DTIM beacons are transmitted by all beacons. Furthermore,present Embodiment 3 calculates an optimal beacon period using thenumber of other access points that are detected, and can therebyseparate beacon transmission timings by access points as far as possiblefrom each other and prevent deterioration of service quality of abroadcast/multicast service in an environment in which a plurality ofaccess points coexist on the same channel.

Embodiment 3 sets a beacon monitoring period immediately be foretransmission of the beacon of the local station, but the presentinvention is not limited to this and the beacon monitoring period mayalso be set immediately after transmission of the beacon of the localstation or the beacon monitoring period may be provided both immediatelybefore and immediately after the beacon of the local station istransmitted.

INDUSTRIAL APPLICABILITY

The wireless LAN communication apparatus and beacon transmission methodaccording to the present invention are especially suitable for use indelivering a broadcast/multicast service, through a plurality of accesspoints, to terminals under the control of the respective access points.

1. An access point apparatus of an infrastructure mode networkcomprising: a receiving section configured to receive a beacon ofanother access point apparatus, the beacon including beacon periodinformation of the another access point apparatus, delivery trafficindication message (“DTIM”) period information of the another accesspoint apparatus, and DTIM count information of the another access pointapparatus, wherein the beacon is a DTIM beacon when the DTIM countinformation is zero and is a non-DTIM beacon when the DTIM countinformation is non-zero; a beacon detection section configured to detectthe beacon received in the receiving section in a predetermined timeimmediately before or after a beacon of the access point apparatus istransmitted; a beacon adjustment section configured to, when a DTIMbeacon of the access point apparatus is determined to collide with aDTIM beacon of the another access point apparatus, adjust a transmissionperiod of DTIM beacons of the access point apparatus by changing a DTIMcount of the next beacon based on the beacon period information, theDTIM period information, and the DTIM count information included in thedetected beacon; and a transmitting section configured to transmit theDTIM beacons at the adjusted transmission period.
 2. The access pointapparatus according to claim 1, wherein, the DTIM beacon of the accesspoint apparatus is determined to collide with the DTIM beacon of theanother access point apparatus when periods of DTIM period informationof the another access point apparatus and the access point apparatusmatch, periods of beacon period information of the another access pointapparatus and the access point apparatus match, and counts of DTIM countinformation of the another access point apparatus and the access pointapparatus are zero, wherein the beacon adjustment section is configuredto, when the DTIM beacon of the access point apparatus is determined tocollide with the DTIM beacon of the another access point apparatus,adjust the transmission period of the DTIM beacons of the access pointapparatus such that the transmission period of DTIM beacons of theanother access point apparatus and the transmission period of the DTIMbeacons of the access point apparatus do not match.
 3. The access pointapparatus according to claim 1, wherein the beacon adjustment section isconfigured to, when the DTIM beacon of the access point apparatus isdetermined to collide with the DTIM beacon of the another access pointapparatus and when a next beacon to be transmitted by the transmittingsection is a DTIM beacon, control the transmitting section to transmitat least one extra beacon other than the DTIM beacon before transmittingthe DTIM beacon to thereby adjust the transmission period of the DTIMbeacons of the access point apparatus.
 4. The access point apparatusaccording to claim 1, further comprising: a measuring section configuredto measure the number of times beacons of the another access pointapparatus are detected in the beacon detection section to be DTIMbeacons, wherein the beacon adjustment section is further configured torefrain from adjusting the transmission period of the DTIM beacons ofthe access point apparatus until the number of times measured by themeasuring section is equal to or greater than a threshold, after whichthe transmission period of the DTIM beacons of the access pointapparatus is adjusted.
 5. The access point apparatus according to claim4, wherein: the beacon detection section is further configured to detectbeacons received in the receiving section both in a pre-transmissionmonitoring time which is a predetermined time immediately before abeacon of the access point apparatus is transmitted and in apost-transmission monitoring time which is a predetermined timeimmediately after the beacon of the access point apparatus istransmitted; the measuring section is further configured to measure thenumber of times DTIM beacons are detected in the beacon detectionsection separately between when a DTIM beacon received in the receivingsection is detected in the pre-transmission monitoring time and when aDTIM beacon received in the receiving section is detected in thepost-transmission monitoring time; and when one of the number of timesDTIM beacons are detected in the pre-transmission monitoring time andthe number of times DTIM beacons are detected in the post-transmissionmonitoring time is equal to or greater than the threshold, the beaconadjustment section is further configured to adjust the transmissionperiod of the DTIM beacons of the access point apparatus.
 6. The accesspoint apparatus according to claim 5, wherein: the beacon adjustmentsection is further configured to set a first threshold to be comparedwith the measured number of times DTIM beacons are detected in thepre-transmission monitoring time and a second threshold to be comparedwith the measured number of times DTIM beacons are detected in thepost-transmission monitoring time to different values.
 7. A beacontransmission method comprising the steps of: receiving a beaconincluding beacon period information comprising information about abeacon transmission period of another access point apparatus, deliverytraffic indication message (“DTIM”) beacon period information of theanother access point apparatus, and DTIM count information of theanother access apparatus, wherein the beacon is a DTIM beacon when theDTIM count information is zero and is a non-DTIM beacon when the DTIMcount information is non-zero; detecting a beacon received in apredetermined time immediately before or after an own beacon istransmitted; when a DTIM beacon of the access point apparatus isdetermined to collide with a DTIM beacon of the another access pointapparatus, adjusting a transmission period of an own DTIM beacon basedon the beacon period information and the DTIM period informationincluded in the detected beacon; and transmitting the own DTIM beacon atthe adjusted transmission period.
 8. The access point apparatusaccording to claim 1, wherein the beacon adjustment section is furtherconfigured to adjust the transmission period of the DTIM beacons of theaccess point apparatus after each detection of a beacon of the anotheraccess point apparatus is detected to be a DTIM beacon.
 9. The accesspoint apparatus according to claim 1, wherein the DTIM count of the nextbeacon is changed by incrementing the DTIM count of the next beacon.